I use a long, unbalanced weapon (Great Long Bardiche), and I have high PS, so I get more than my fair share of TKs. 90% of them are because I'm swinging at an enemy, but someone else kills them mid-swing, so my swing goes right through them and hits my teammate. I can't just block to end the swing, because my weapon is unbalanced. I apologize in this situation, because I chose an unbalanced weapon and accept the consequences.
The rest of the TKs usually go something like this; I'm wailing on some guy's Huscarl shield, trying to bring it down. When it finally breaks, a facehugger on my team swoops in front of me with his Steel Pick or something to get the kill. If you deliberately step in front of me while I'm engaged with an enemy and you get smacked in the butt by my Bardiche, I don't apologize. Just like I accept that my choice of weapon will cause me to deliver more TKs, facehuggers need to understand that their tactics get them TKed.
This is a team game. TKing hinders your team's chance to win, which is enough penalty. I don't support multiplier/reflective dmg/gold/etc loss upon TKing, because this unfairly punishes people with unbalanced weapons and those that happen to deliver a TK because of their own teammate's idiocy. It's not always the TKer who is at fault.
P.S.-The above only applies to non-intentional TKs. People that spawnhit should get insta-kicked from the server.